Plasma accelerator



Oct. 16, 1962 w. w. sALlsBu-RY 3,059,149

PLASMA ACCELERATOR Filed Feb. 12, 195s '4 sheets-sheet 1 Oct. 16, 1962 w. w. sALlsBURY 3,059,149

PLASMA ACCELERATOR Filed Feb. l2, 1958 4 Sheets-Sheet 2 il III,"

Oct. 16, 1962 w. w. sALlsBURY 3,059,149

' PLASMA ACCELERATOR Filed Feb. 12, 195s 4 sheets-sheet s Oct. 16, 1962 w. w. sALlsBuRY' PLASMA AccELERAToR Filed Feb. 12, 1958 4 Sheets-Sheet 4 Phase 3,059,149 PLASMA ACCELERATR Wneld W. Salisbury, Palo Alto, Calif., assigner to Zenith Radio Corporation, a corporation of Delaware Filed Feb. 12, 1953, Ser. No. 714,719 14 Claims. (Cl. 315-267) This invention relates to a plasma yaccelerator having a wide range of applications. It is useful, for example, in the study of shock-wave phenomena, stagnation temperatures, aerodynamic heating effects and the like. In fact, it has unusual versatility, being further suitable for use in such remotely related applications as controlled thermonuclear reactions, cutting torches, and object or ship propulsion.

Plasma accelerators, as such, are known in the art and are premised on the proposition that a closed electrical system carrying a current develops a force tending to increase the energy stored in the system. This energy is a function of current intensity and is an integration of the flux lines resulting therefrom so that the effect under consideration is to increase the volume or space enclosed by the conductive system. Translated into observable physical phenomena, this effect manifests itself in a tendency of the conductive system to expand. If the current path is completed through a space discharge across a gap, a plasma of conductive particles is created at the gap and less force is generally required to separate the plasma from the electrodes than to displace the conductors which are normally restrained from movement by means of their mechanical supports. Accordingly, the force developed in the system expends itself by detaching the plasma.

One form of accelerator operating on this principle comprises a conductive system that may be completed through a spark gap and further comprises a pair of linear parallel conductors in conductive connection with the gap electrodes and extending therefrom to delineate a desired acceleration path for the plasma. When the gap is broken down in response to the application of a suflicient potential difference to its electrodes, current of high intensity flows through the first-mentioned system and the force developed in the system detaches the plasma and projects it along the second-mentioned system. Thus, acceleration of the plasma commences and it persists so long as the force of the circuit is sustained; in other words, so long as current iiows in the first system. Such current iiow is possible, however, only so long as the circuit is completed through the plasma, e.g., only so long as the plasma maintains contact with the conductors defining the acceleration path. Since it is diflcult to preserve this necessary contact condition, the apparatus in question is not reliable.

Another type of accelerator known in the art employs a plasma source which supplies a plasma having charged particles of one polarity only and the plasma is subjected to an intense electric field which produces its acceleration. A re-entrant type of magnetic system returns the plasma to the electric eld repeatedly to achieve a desired final acceleration. Since electric fields exert opposing effects on charged particles of different polarities, this apparatus is inherently restricted to operating upon plasmas of charged particles of one polarity. Such plasmas are subject to space-charge effects within themselves which result in less dense plasmas than required for certain applications, for example, in controlled thermonuclear reactions.

It is an object of the present invention, therefore, to provide a plasma accelerator which avoids the deficiencies and limitations of such prior art apparatus.

lt is a further object of the invention to provide im- 3,59,l49 Patented Oct. 16, 1962 proved apparatus for accelerating plasmas to high velocities along linear acceleration paths.

Another object of the invention is the provision of improved and simplified plasma accelerating apparatus suitable for use in a Wide variety of unrelated fields of application.

A plasma accelerator, constructed in accordance with the-invention, comprises an evacuated structure defining an acceleration path and means for establishing in a predetermined region of the structure a plasma including charged particles of both polarities. The accelerator includes a magnetic traveling wave generator having a plurality of magnetic field coils spaced along and encircling transverse sections of the acceleration path. There are means for energizing those coils to establish magnetic field components along the path as well as means for timing the energization of the `coils with respect to one another so that such field components conjointly create a magnetic wave traveling along the path and accelerating the plasma. iFinally, means are disposed along the path for accommodating an element in a position to be bombarded by the accelerated plasma.

The foregoing and other objects of the invention, together with further advantages and benefits thereof, will be more clearly understood from the following description of particular embodiments thereof taken in conjunction with the annexed drawings in the several figures of lwhich like components are designated by similar reference characters and in which:

FIGURE l is a schematic representation of a plasma accelerator embodying the invention;

FIGURE 2 is a schematic representation of an apparatu-s similar to that of FIG. l but arranged for acceleration of a continuous or sustained plasma as distinguished from a plasma pulse;

FIGURE 3 represents a modified form of the coil arrangement of the apparatus of FIG. 2;

FIGURES 4, 5 and 6 are segmented views illustrating modifications that may be made to the apparatus of FIGS. l and 2;

FIGURE 7 represents schematically `apparatus constructed in accordance with the invention and adapted to control thermonuclear reaction;

FIGURE 8 is a modification of the arrangement of FIG. 7, having a feed-back or re-entrant feature;

FIGURES 9-l0, ll-lZ, and 13-1-4 show further arrangements of plasma accelerators featuring a non-linear path of travel; and

FIGURE l5 is a detail pertaining to these last-mentioned arrangements.

Referring now more particularly to FIGURE l, the accelerator there represented comprises an evacuated structure -10 which may be a long tube of insulating material enclosing an evacuated space and dening thereby a linear acceleration path. Fused quartz and other refractory materials may be used as the accelerating tube so long as the electric conductivity does not impair the driving action of the magnetic coils to be described presently. Expressed differently, the conductivity of the tube must not be high enough to support currents within the tube which would shield the plasma from the driving effects of the magnetic coils. A vacuum pump 11 is in communication lwith the tube by means of a valve and conduit y12r to establish and maintain a desired vacuum condition therein. It is appropriate in many applications of the accelerator to have the vacuum in the order of 10*5 millimeter of mercury.

The apparatus under consideration may, for convenience, be considered as a hypersonic wind tunnel for studying aerodynamic heating effects and/or operating characteristics of missiles or ships. For that application,

structure l0 has a loading section -13 providing access to the chamber through the usual vacuum-sealed connection. If the behavior of an element such as a missile or ship is to be studied, a scale model thereof is suspended within the section of the structure to which access is had through port 13.

The apparatus has means for establishing in a predetermined region of the evacuated structure a pulse of plasma including charged particles of both polarities, that is, an essentially neutral plasma with respect to electric charge. A variety of mechanisms may serve this purpose and most of them are well known to the art. An independent plasma source may inject a pulse of plasma into the leading section of the acceleration path or a small burst of suitable gas may be suddenly introduced into the structure by means of a quick operating valve. It is also understood that a rapid change of the magnetic field in the leading section of the acceleration path will ionize the low temperature residual gas to form a plasma. A very simple mechanism for establishing the plasma employs spark-gap electrodes and a valve for admitting a puff of gas into the gap region so that the electrode potential causes the gap to break down. For convenience of illustration, however, the apparatus employs a gap having electrodes and 16 positioned at t-he leading section of tube 10, leading in respect to the direction of the plasma acceleration which is indicated by an arrow, and reliesupon establishing a sufficiently high potential across the electrodes to ionize residual gas and create the plasma in situ. Since a pulse of excitation potential is applied to these electrodes, a pulse of plasma is created.

Acceleration of the plasma is accomplished by a moving magnetic field, taking advantage of the kno-wn fact that a conductive body, including a plasma of ionized gas, may be accelerated to great velocities through the mechanism of such a field. Accordingly, the apparatus includes a magnetic traveling wave generator having a plurality of exciting coils spaced along and encircling transverse sections of tube 10 and the acceleration path. The exciting coils may have equal axial lengths or their lengths may increase with the separation or distance of the coil from the spark electrodes. FIG. l represents the casein which the coils 17, 17', 17, 17n have equal lengths. Their spacing with respect to one another is small enough that the field intensity along the structure is substantially uniform, giving consideration to 4the fact that the field contributions of contiguous coils combine vectorially. The number of coils employed is determined by the length of the accelerating path and, of course, the length of tube 10. That tube is represented of indefinite length in the drawing since any practical embodiment of the invention would include a much longer acceleration path than can be readily illustrated. The conductor from which the coils are made is not critical as to electrical properties except that the coil circuits are to have particular frequency characteristics determined by their inductance and capacitance and the coil design must permit these characteristics to be realized.

There are means in the apparatus for energizing the several coils to establish magnetic field components along the yacceleration path. Generally, and especially Where the apparatus is pulsed rather than arranged for continuous operation, the energization of the coils is derived from condensers charged to a high potential in the interval between pulses and discharged rapidly through the coils in each pulse interval. The condensers have been designated 18, 18', 18, 1%n and are charged from a high voltage D.C. power supply 14 of conventional design. The power supply has a number of output terminals each of which is connected through a charging resistor 19, 19', 19, 19n to an associated one of the condensers. Three such connections have been shown in the drawing but the others have been omitted to simplify the drawing. It will -be understood, however, that each of the resistor terminals T-1, TeZ etc. is conductively connected to the correspondingly identified terminal of supply 19. The discharge circuit from each condenser to its associated coil is normally interrupted by means of a spark gap interposed therebetween. The spark gaps are designated 20', 20', Ztl, 24), One of a series of electrodes 21, 21', 21", 21n is associated with each such gap to break the gap down and complete the energizing circuit for the coil in response to the application of -a trigger pulse to such electrode.

The application of trigger pulses to the spark electrodes is under the control of means for timing the energizing of the coils both with respect to the creation of the plasma pulse and with respect to one another. The timing means or circuit 22 may also be of conventional design and construction. Any electronic timing device arranged to apply output pulses to a plurality of channels with a desired time relation to one another will be adequate. An electronic ring circuit is one example of such a timer. Another well known yand suitable device is in the form of a pulse generator applying a pulse of potential having a well defined duration to a multiple-tap delay line. An output pulse is derived at each tap along the line and the relative positions of the taps fixes the relative timing of the several output pulses. Each output terminal of the timing circuit connects through an associated one of a series of pulse amplifiers 23, 23', 23, 23n to an assigned one of trigger electrodes 21, 2l etc. A further output terminal of the timing circuit is connected through another pulse amplifier 24 to gap electrode 15' at the leading end of tube w.

In considering the operation of the apparatus, it will be assumed that the object whose performance is to be observed has been positioned within the final section of tube 10. It will be assumed further that the desired vacuum condition has been established within the tube and that condensers 18, 18 etc. have been charged by supply 14. When timing circuit 22 is actuated, a first output pulse, after amplification in amplifier 24, establishes a sufficient potential difference across electrodes 15, 16 to break the gap down and create within tube 10 a plasma of charged particles, such as ions, of both positive and negative polarity. Directly thereafter, another output pulse is applied through amplifier 23 to gap electrode 21 to break down gap 20 and discharge condenser 18 through coil 17 and establish one component of a magnetic field in the leading section of tube 1). In like fashion, succeeding output pulses from timing circuit 22 cause successive energization of the remaining coils in such time relation to one another that their individual field components conjointly create a magnetic field Wave traveling along the acceleration path and accelerating the pulse of plasma along that path. The magnetic eld variations which propel the plasma maintain the temperature and ionization of the plasma and may even increase them as the acceleration proceeds. There will be some slip between the plasma and the moving magnetic field or traveling magnetic wave but this slip decreases as the plasma is accelerated and reaches a minimum value determined by the conductivity of the plasma, its mass, and the viscosity of the residual gas, if any, in which the plasma moves. Having attained the minimal slip value, the plasma progresses along the acceleration path with a velocity only slightly less than that of the propelling magnetic field. Plasma velocities of the order of 105 to l08 centimeters per second and even greater may be produced with proper choice of coil size, resonant frequency of the coil circuit-s and gas pressure within tube 10.

Whenever any of the condensers 18, 18 etc. is discharged through its associated coil, the current and the ensuing magnetic field component are of sinusoidal waveform having a frequency corresponding to the resonant frequency of the coil circuit. Frequencies in the range of l0 megacycles will produce very high velocities. Preferably the coil circuits, ignoring the loading imposed by the conductive plasma, have a high Q or figure of merit.

The number of coils required to complete a phase pattern or a complete cycle at the operating frequencies constitutes a geometrical phase wave length. In preferred operation, the phase length 7x embraces six of the coils, as indicated in FIGURE l, so that the apparatus is analogous to a six pole motor structure. 'Ihe larger the number of poles, the more smooth is the transition of the lield from pole to pole, the more uniform is the field, and the more uniform is the acceleration of the plasma. The phase wave length multiplied -by the frequency gives the magnetic velocity and the plasma velocity is the difference between the magnetic velocity and the magnetic velocity multiplied by the slip. These relations may be expressed in the following manner:

(l) Vmf=1pf (2) VPL-(1 5) Vm where:

Vm is the velocity of the magnetic ield kp is the geometrical phase wave length f is the exciting frequency sis the slip The slip may vary from unity to zero but decreases as the plasma is accelerated. The plasma frequency is equal to the exciting frequency times the slip as indicated in the following expression:

There is considerable leeway in the timing of the traveling magnetic wave at the start of the acceleration path relative to the breakdown of the gap between electrodes 15, 16. The timing may be such that coil 17 is excited just as the plasma passes the center line of the coil but it may be found more expedient to adjust the timing empirically for maximum acceleration. If coil 17 is excited before the plasma has reached the central position of that coil, the iield of the coil tends to decelerate the plasma. In certain installations this may be highly desirable at least so far as the timing of the `first coil is concerned. The deceleration may eifect bunching and increase the density of the plasma. At the other extreme of the timing range is the fact that the timing may be delayed so long that the eld contribution of the coil adds no increment to the propelling force exerted on the plasma. The tield of each coil falls off fairly fast in the yaxial direction of the tube so coil 17, for instance, probably has little if any influence in the region of coil 17 Therefore, coil 17 should be fired certainly well before the plasma reaches that part of the tube encircled by coil 1'7".

The magnetic traveling wave may have a constant velocity along the acceleration path or it, too, may be accelerated. Where the spans of the coils are the same and their exciting circuits have the same time constant and are triggered with pulses having the same time separation from one to the next, the traveling Wave has constant velocity.

Acceleration of the magnetic wave in embodiments where the coils are of uniform axial length requires that the time constants of the several energizing circuits decrease with separation or distance of the driving coils from gap electrodes 1S, 16. There must also be a corresponding decrease in time separation of the successive trigger pulses applied to electrodes 21, 21 etc. Where the coils have an increasing axial length along tube 10 from electrodes I5, 16, acceleration will result with equal time constants of the coil exciting circuits and uniform separation of the trigger pulses. It may likewise result but with a greater rate of acceleration, if the time constants of the exciting circuits decrease with the distance of the coils from electrodes 15, 16 provided that there is a corresponding decrease in the time separation between trigger pulses.

The change in axial length of the coils may be accomplished in any of several ways. The number of coil turns may be increased, the inter-turn spacing may be increased or both may be increased. Moreover, the

t' number of turns and interturn spacings may be kept constant but the conductor width may be increased from turn to turn.

The plasma as finally accelerated enters the terminal portion of tube 10 including the object or missile under observation and the tremendous velocities imparted to the plasma permit study of the aerodynamics of the object.

The arrangement just described employs pulse techniques but the accelerator of the invention is equally useful for continuous acceleration. A continuous accelerator is shown schematically in FIGURE 2 wherein it will be seen that the lengths of the exciting coils, measured along the axis of tube 10, increase with distance from electrodes 15, 16. This coil feature takes advantage of the fact that more ecient acceleration is produced if the magnetic wavelength kp is small at the start of the path and increases with the acceleration. For continuous acceleration, there must be a sustained plasma which may be supplied through any known plasma source or may result from a sustained breakdown of the gap between electrodes 15, 16. Especially is this so Where the apparatus uses what are known as loaded electrodes. These are electrodes formed of titanium, for example, and characterized by having an adsorbed charge of a hydrogen isotope; that is, the electrodes have been subjected to an atmosphere of deuterium or tritium and have retained considerable amounts of the gas. An arc drawn between such electrodes releases the gas in plasma form but yet the electrodes do not surrender the gas itself to the influence of the vacuum pump connected to tube 10.

The continuous accelerator uses a S-phase oscillator Sil coupled to 3-phase buses A, B and C. The connections of the coils for a 6-pole arrangement, where six pole has the same connotation as explained in the discussion of FIGURE l, is demonstrated by the wiring plan of FIGURE 2. In this environment, the coils are excited by sustained alternating currents having such phase relations, with respect to one another, that the necessary traveling magnetic wave results. The current and magnetic ield component of each coil have a phase displacement of 60 degrees with respect to its neighbor.

The continuous accelerator of FIGURE 2 operates in generally the same way as that of FIGURE 1 and may work at atmospheric or greater pressures so long as sufficient power is available from generator 30 to propel the plasma.

It has been indicated that smooth transitions from one exciting coil to the next yield best results, certainly more uniform results with the apparatus. Interlacing of the iield coils permits the smoothest transition particularly if the interlacing is on the basis of coil turns. Such an interturn interlace pattern, for the 6-pole, 3-phase coil arrangement of FIGURE 2, is represented in the developed plan View of FIGURE 3. The first coil is between terminal portions 17 and 17a. The second coil has terminal portions 17 and 17'a. The terminal portions for the next succeeding coils are: 17" and 17"a; 171V and 171Va; 17V and 17Va and finally 17VI and 17Va.

A continuous axial magnetic ield may be employed to guide the plasma and prevent it from losing heat to the walls of tube 10 or perhaps even melting the tube or, in the ultimate, function as a wave guide so that the tube walls may be dispensed with altogether. FIGURE 4 represents a small axial section of tube 10 with a solenoid 33 mounted in concentric relation with exciting coils 17, @17' etc. for creating such an additional magnetic iield. The field is of substantially constant intensity, assuming that the winding carries a direct current of constant amplitude. In net effect, this superposed iield confines or constricts the plasma transversely of theacceleration path. If the iield is increased in intensity at portions of the acceleration path remote from electrodes 15, 16 the accelerated plasma stream may be converged to attain a higher density and/or temperature. Increase in plasma density is possible because there is no significant space charge effect present in the plasma. A layer 34 of insulating material is included between coils 17, 17 etc. and winding 33. Obviously, it must have suitable perforations to permit the terminal portions of the exciting coils to pass out of the structure between turns of winding 33.

FIGURES and 6 represent a further modification that may be made in the evacuated structure. As here represented, there is a second tube a disposed coaxially within tube 10 and formed of the same insulating material. Collectively, they provided an acceleration space in the form of a hollow cylinder. Tube structure ltr and the exciting coils tmay be enclosed within a body 3S of silicon steel or, at high operating frequencies, the enclosing material may be a magnetic ceramic, such as that known as Ferrite The inner tube lila would be filled with the same material. This added magnetic material may improve the magnetic field strength and distribution provided that the plasma desired is tenuous enough that the required magnetic pressures do not imply magnetic saturation. The external enclosing material 35 may, if desired, take the form of laminat-ions positioned with their width dimension extending radially to the structure as shown in FIGURE 6. Apparatus having the modification represented in FIGURES 5 and 6 is useful for accelerating large quantities of plasma in which case the accelerating tube 10 has a diameter much larger than the skin depth in the plasma at the plasma frequency.

A plasma accelerator may be embodied in apparatus for achieving -a controlled thermonuclear reaction. One arrangement designed to that end is illustrated schematically -in FIGURE 7. It has a central and enlarged section 40 which may be considered a reaction space and that section is interposed between two linear section 1G 'and 10". Each of these sections, considered individually, is constructed in generally the `same way as the linear accelerator of FIGURE l and it is not necessary to repeat the details. Suce it to say tha-t the gap electrodes included in each section are at substantially equal distances from reaction space 40 and also the coils in one section correspond in spacing, number of turns, time constant etc. to a like coil in the other section. Since there is identity of structure in accelerators 10 and .10, it is not necessary to have duplicate timing circuits. Instead, each energizing circuit includes one coil of one accelerator in series with the corresponding coil of the other accelerator. This is illustrated by the connection 41 interconnecting the lead coils of both accelerators. Similiar connections will be made (although not shown in the drawing) between the terminals of iike designation, such as Tb to Tb, Tc to Tc, TI1 to Tn etc. The condensers 1S, I8 etc. when discharged simultaneously excite two coils, one located in each accelerator. Moreover, the plasma forming electrodes of the two sections share a common trigger circuit as indicated by the connection 42. Each accelerator 10 and 10 operates in the same way as the accelerator of FIGURE l to accelerate a plasma to the reaction space 40. The accelerators are in coaxial alignment and t-he plasmas travel to the reaction space with substantially identical acceleration because of the symmetry of the accelerators and their concurrent actuation.

Where the apparatus of FIGURE 7 produces a collision of two plasmas in space 40, very high stagnation temperatures are developed. Stagnation temperature is the temperature attained by the plasma under the influence of adiabatic compression and it is desirable to study such temperatures in different media, especially those of high Mach numbers. The Mach number is a function of the velocity of the plasma to the velocity of sound and is a phenomena that must be studied in order to learn the behavior of space missiles in the reentrant part of their flights. The apparatus of FIGURE 7 lends itself to such studies.

This apparatus further lends itself to the field of controlled thermonuclear reaction as will be apparent from the following considerations. Each section 1W and 10" accelerates a plasma to reaction space 40, as explained, and the plasmas have essentially -t-he same speed relative to the evacuated structure, assuring their meeting within the reaction space. The collision of elements is, in fact, an impact of two electrically neutral clouds or masses of electrons and atomic nuclei. Electrical neutrality is assured by the strong electrostatic forces which appear as soon as the light electrons attempt -to outstrip and separate from the heavy nuclei. Such considerations, however, do not prevent the light nuclei of the reactant (with their electrons, to preserve charge neutrality) from outstripping heavier contaminant nuclei (with their electrons) so that a purication effect exists-the desired nuclei reach the reaction space before the contaminants. Further, since the electrons and nuclei travel at substantially the same velocity, the stagnation temperature of the electrons will be much less than the stagnation temperature of the heavy nuclei, thereby avoiding excessive loss of energy by radiation from the electrons. The collision produces a high temperature shock wave, having a sufficiently high stagnation temperature to result in a nuclear reaction especially if the plasma is formed of dueterium ions or ions of other gases such as tridium which may yield most readily to therrnonuclear reaction.

Two essential conditions must be satisfied if a thermonuclear reaction is to result: (a) the density of the impacting plasmas must be sufficient that there is a high probability of ion collision and (b) the velocity of the plasmas must be great enough to create a stagnation temperature of the order of millions of degrees Kelvin. Both conditions can be met with the apparatus of FIGURE 7. It may be desirable to add a solenoid winding encircling each section 10 and d in a region close to reaction space 40 to increase the density and temperature of the plasmas prior to their entry into the reaction space as discussed in connection with FIGURE 4.

Apparatus of the type represented in FIGURE 7 may likewise be made continuous in operation through the expedient of feedback or re-en-try. A modification having this feature is shown in FIGURE 8. Essentially a third linear accelerator is provided, leading from the reaction space and feeding back to the leading sections of each `accelerator I0 and IG". While a single accelerator may be employed with a T-termination leading to units 10 and lll, it is convenient and preferred to construct the third accelerator as as a 2-unit device, having one section leading from the reaction space to unit 10 and another section leading from that space to unit I0". In the figure, the first section of the feed back accelerator is designated 50 and the other `section is 5l. The accelerators 50 and 5l have generally the same construction as accelerators 10' `and 10 but are proportioned to achieve low velocity for a large Volume of plasma translated by each.

The embodiment of FIGURE 8 is a further demonstration of 4the wide versatility of the plasma accelerator which is here used in the role of a vacuum pump. Charged gas ions constituting the plasma will be certainly present in reaction space 4t?, and accelerators Sti, 51 are effective to draw the gas out of the space for any purpose, not necessarily restricted to the feed-back function assigned to those accelerators in this particular embodiment.

It is well known that a change in a strong magnetic eld which influences an element such as a gaseous medium or atmosphere effects ionization. The leading section or coil of an accelerator embodying the invention may exert such a field change and effect ionization of any residual gas within the space in which the field of the coil penetrates. It is clear, therefore, that the accelerator may have wide utility `as a vacuum pump.

In each of the several embodiments of the invention thus far described, evacuated structure l0 defines a linear path `along which the plasma is accelerated but it is to be understood that the teachings are not confined in application to linear structures. It is appropriate and in many instances highly `desirable to effect acceleration of the plasma along la curvilinear path and this may be accomplished by the sa-me general type of structure described hereinabove with the addition of a suitable vehicle to assure that the plasma is not permitted to impact against the walls of the structure defining its path of travel. Representative forms of nonlinear or rotary plasma accelerators are shown in FIGURES 9-l0, ll-lZ, and 1'3-14. For simplicity, the excitation circuits of the coils for developing a traveling magnetic wave, the triggering and timing mechanisms, the plasma electrodes and vacuum systems have been omitted. They may be course be generally similar to the arrangements shown and described in the other figures of this specification.

The embodiment of FIGURES 9-l0 is one wherein evacuated structure 16 is in the form of a toroid having exciting coils 17, 17' 17 spaced therealong in order to create a magnetic Wave traveling along the path defined by the toroid. Since in most applications it will be found desirable to have the plasma execute several passes or traverses of the toroid in arriving at final or maximum velocity, it is preferable that the exciting coils be energized from an alternating current source such as a three-phase source like that discussed in connection with the embodiment of FIGURE 2. The relative phases of excitation of the field coils is indicated by appropriate legends in FIGURE l0.

A series of magnets 60, 6i) are likewise positioned along structure to contribute a field for the purpose of defiecting the plasma away from the outer Wall of the toroid. While permanent magnets may be employed for this purpose, it is convenient to utilize an electromagnet of the type shown in FIGURE l5. It has a magnetic structure 61 which is E-shaped and an energizing coil 62 wound about the central leg to create a magnetic field having a fiux pattern indicated in broken construction lines. A D.C. current is fed to coil 62 and its strength is adjusted to effect a field penetration of the toroid to deflect the plasma away from the conlining walls. The magnets 60, 60 must be sufficiently close to one another that the protective magnetic field is able to steer the plasma throughout the entire path of the toroid and successive ones of the magnets are to be oppositely poled, that is, they have opposite polarity.

In considering the operation of the arrangement of FIGURES 9-l0, the curved nature of evacuated structure 10 and the function of magnets 60, '60' will be momentarily ignored. Field coils 17, 17' 17I1 create a magnetic wave traveling the path of the toroid and the gas plasma established in situ within the toroid or injected therein is accelerated by that magnetic traveling wave. 'Recognizing that the structure shown is circular, it is apparent that the acceleration of the plasma is in the direction to cause it to impinge upon the confining walls of the toroid. This is obviated by the steering or directing field contributed by magnets 60, 60 which imparts a force to direct the plasma clear of the walls of the toroid. The diameter of the toroid may be sufficiently large that the plasma achieves maximum velocity in a course of a single traverse of the path but the structure under consideration permits operation such that several passes are required to bring the plasma to final velocity. This has the distinct advantage of reducing the physical size of the structure and is entirely feasible s-o long as field coils 17, 17 17n have a sustained excitation. Two opposing forces are exerted upon the plasma in its path of travel: (l) a centrifugal force and an opposing steering force established by magnets 6G, '60. The strength of the steering field is adjusted to obtain a balance between these forces in order that the plasma may be directed along a mean path intermediate the confining walls 4of the toroid. It should be pointed out that the 4balance condil@ tion between the centrifugal force and the steering force of magnets 60, 60 compresses the plasma and increases its density.

Rotary accelerators of this type are adapted to many installations. For example, a quick-acting valve mechanism (not shown) may be associated with the toroid to inject a pui or measured quantity of neutral or unionized gas into the toroid to impact the accelerated plasma which has attained maximum velocity. Operated in this fashion, the device may serve as a neutron generator.

Another use of the structure which readily suggests itself is a thermonuclear applicati-on of the type described in connection with FIGURE 8. The evacuated structure there represented has the configuration of a figure 8 and may be thought of as two loops with an interposed reaction space in communication with each. Considering each loop as a rotary plasma accelerator and operating them in complimentary fashion, results in the acceleration of two plasmas with like velocity and controlled density to the reaction space to effect a collision in the reaction space as described in connection with the operation of the embodiment of FIGURE 7.

The embodiment of FIGURES ll-l2 differs from that of FIGURES 9-10 in that the evacuated chamber 10 has the form of a sphere. The exciting coils 17, 17 and 17 encircle the sphere and are driven by a threephase supply in the manner of FIGURE 2. Magnets 6ft, `60 are disposed about the periphery of the sphere for the purpose of providing a steady magnetic field to steer the plasma away from the walls of the chamber.

The nal embodiment of FIGURES 13-14 is one in which the evacuated chamber 10 is rath-er like a flattened sphere having a generally elliptical cross-section as indicated in FIGURE 13. Its coil arrangement and the disposition of steering magnets 60, 60 may be generally the same as that of the structures of FIGURES 9-1() and ll-12.

Any of the described embodiments of the invention serving as a continuous accelerator may accomplish propulsion of an object carrying the unit. Thrust is a product of mass times velocity and the velocities possible with the accelerator should result in a thrust useful for propulsion. It is also apparent that the accelerator projects a plasma of very high temperature. If the trailing end of the acceleration path is transparent to the plasma and if the accelerator is of the continuous variety, the apparatus may be employed as a torch or for use in brazing or welding.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall Within the true spirit and scope of the invention.

I claim:

1. A plasma accelerator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of magnetic field coils spaced along and encircling said path; means for energizing said coils to establish magnetic field components along said path; means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field wave traveling said path and accelerating said plasma therealong; and means disposed along said path for accommodating an element in a position to be bombarded by the accelerated plasma.

2. A plasma accelerator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of interlaced magnetic field coils spaced along and encircling said path; means for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field Wave traveling said path and accelerating said plasma therealong; and means disposed along said path for accommodating an element in a position to be bombarded by the accelerated plasma.

3. A plasma accelereator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of magnetic field coils having interlaced coil turns spaced along and encircling said path; means for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field Wave traveling said path and accelerating said plasma therealong; and means disposed along said path for accommodating an element in a position to be bombarded by the accelerated plasma.

4. A linear plasma accelerator comprising: an evacuated structure, including a pair of linear and coaxial elements defining an acceleration path in the form of a hollow cylinder; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling Wave generator including a plurality of magnetic field coils spaced along and encircling said path; means for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field wave traveling said path and accelerating said plasma therealong.

5. A linear plasma accelerator comprising: an evacuated structure, defining a linear acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling Wave generator including a plurality of magnetic field coils spaced along and encircling said path; means for energizing said coils to establish magnetic field components along said path; means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field Wave traveling said path and accelerating said plasma therealong; and means, including a solenoid mounted in concentric relation with said exciting coils, for creating an additional magnetic field having a fiux density along said path selected to conne said plasma transversely of said path and to control the density of said plasma.

`6. A plasma accelerator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a pulse plasma including charged particles of both polarities; a magnetic traveling Wave generator including a plurality of magnetic field coils spaced along and encircling said path; means for supplying pulsed energy to said coils to establish magnetic Ifield components along said path; means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field wave traveling said path and accelerating said plasma therealong; and means disposed along said path for accommodating an element in a position to be bombarded by the accelerated plasma.

7. A plasma accelerator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a sustained plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of magnetic coils spaced along and encircling said path; means for supplying sustained alternating current tolsaid coils to estaablish magnetic field components along said path; means for phasing the energizing current of said coils with respect to one another so that said field components conjointly create a magnetic field wave traveling said path and accelerating said plasma therealong; and means disposed along said path for accommodating in element in a position to be bombarded by the accelerated plasma.

8. A plasma accelerator comprising: an evacuated structure defining an acceleration path; means for establishing in a predetermined region in said structure a pulse plasma including charged particles of both polarit1es; a magnetic traveling wave generator including a plurality of magnetic field coils spaced along and encircling said path; means for supplying pulsed energy to said coils to establish magnetic field components along said path; and means for adjusting the timing of the energization of said coils with respect to the establishment of said plasma pulse to control the density thereof and with respect to one another so that said field components conjointly create a magnetic field Wave traveling said path and accelerating said plasma therealong.

9. A linear plasma accelerator comprising: an evacuated structure defining a linear acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of magnetic field coils of substantially equal axial length spaced along and encircling said path; energizing circuits, individual to said coils and having a time constant that decreases with the distance of the coil from said region, for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coil circuits with respect to one another so that said field components eonjointly create a magnetic field wave traveling said path With an acceleration determined by the change of time constant of said coil circuits and accelerating said plasma therealong.

l0. A linear plasma accelerator comprising: `an evacuated structure, defining a linear acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling wave generator including a plurality of magnetic field coils spaced along and encircling said path and having axial lengths which increase With the distance of said coils from said region; energizing circuits, individual to said coils and having a time constant that decreases with the distance of the coil from said region, for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coil circuits with respect to one another so that said field components conjointly create a magnetic field wave traveling said path with an acceleration determined by the change of time constant of said coil circuits and accelerating said plasma therealong.

11. A linear plasma accelerator comprising: an evacuated structure, defining a linear acceleration path; means for establishing in a predetermined region in said structure a plasma including charged particles of both polarities; a magnetic traveling Wave -generator including a plurality of magnetic field coils spaced along and encircling said path and having axial lengths which increase With the distance of said coils from said region; energizing circuits, individual to said coils and having substantially the same time constant, for energizing said coils to establish magnetic field components along said path; and means for timing the energization of said coils with respect to one another so that said field components conjointly create a magnetic field Wave traveling said path and accelerating said plasma therealong.

l2. A plasma accelerator comprising: an evacuated structure defining an acceleration path having two sections and an interposed reaction space; means for establishing in a region of each such section, equi-distant from said reaction space, a plasma including charged particles of both polarities; a magnetic traveling wave generator including two like pluralities of magnetic eld coils spaced in corresponding fashion along and encircling said sections, respectively; means for energizing said coils of both pluralities of coils to establish magnetic iield components along said path; and means for simultaneously actuating said energizing means of both said plurality of coils concurrently and for timing the energization of said coils in each said plurality with respect to one another so that said ield components conjointly create a pair of magnetic eld Waves traveling and accelerating said plasmas along each said section of said structure in the direction of said react-ion space.

13. A plasma accelerator comprising: an evacuated structure defining an acceleration path having two sections and an interposed reaction space; means for establishing in a region of each such section, equi-distant from said reaction space, a plasma including charged particles of both polarities; a magnetic traveling Wave generator including two like pluralities of magnetic iield coils spaced in corresponding fashion along and encircling said sections, respectively; a series of energizing circiuts, each including one coil of one of said pluralities and the corresponding coil of the other said plurality in series, for energizing said coils to establish magnetic iield components along said path; and means for actuating said circuits in timed relation with respect to another so that said iield components in each said sect-ion conjointly create a magnetic iield Wave traveling toward said reaction space and `accelerating said plasmas to said space with substantially identical acceleration.

14. A plasma accelerator comprising: an evacuated structure dening an acceleration path having two sections yand an interposed reaction space; means for establishing in a region of each such sections, equi-distant from said reaction space, a plasma including charged particles of both polarities; a magnetic traveling Wave generator ineluding two like pluralities of magnetic field coils spaced in corresponding fashion along and encircling said sections, respectively; means for energizing said coils of both pluralities of coils to establish magnetic ield components along said path; means for (simultaneously) actuating said energizing means of both said plurality of coils concurrently and for timing the energization of said coils in each said plurality with respect to one another so that said iield components conjointly create a pair of magnetic field waves traveling and accelerating said plasmas along each said section of said structure in the direction of said reaction space; and meansv for feeding back plasma from said reaction space to the leading portion of both said sections of said evacuated structure.

References Cited in the le of this patent UNITED STATES PATENTS 2,232,030 Kaliman Feb. 18, 1941 2,489,082 De Forest Nov. 22, 1949 2,545,595 Alvarez Mar. 20, 1951 2,683,216 Wideroe July 6, 1954 2,692,351 Morton Oct. 19, 1954 2,804,511 Kompfner Aug. 27, 1957 2,819,423 Clark Jan. 7, 1958 2,867,748 Von Atta Jan. 6, 1959 2,898,508 Mallinckrodt Aug. 4, y1959 2,899,598 Ginzton Aug. 11, 1959 FOREIGN PATENTS 640,910 Great Britain Aug. 2, 1950 707,271 Great Britain Apr. 14, 1954 OTHER REFERENCES Article by Thonemann et al., pp. 34 and 35, Nature for Jan. 5, 1952, vol. 169, No. 4288.

Nucleonics, vol. 12, No. 12, December 1954, pages and 41. 

